Described below is a hydrodynamic boil bearing assembly for use in rotating machinery.
Hydrodynamic foil bearings have been suggested to improve the performance of ordinary hydrodynamic bearings for high speed rotating systems. The principle of operation of foil bearings is based on formation of a pressurized fluid film in the annular gap between an outer periphery of a rotating shaft and a bearing surface formed by either a plurality of foils or a single bump foil. This pressurized fluid film works as lubrication layer, reducing frictional force between the outer periphery of the rotating shaft and the inner wall surface of the foil during operation and allows the bearing to effectively support a high load. Due to this pressurized fluid film, the rotating shaft rotates while floating in the air and maintains a predetermined distance from the foil. The pressurized fluid film in the annular gap between the outer periphery of the rotating shaft and the inner wall surface is formed when the rotating shaft rotates at a certain speed.
A potential advantage provided by the foil bearings over the hydrostatic bearings is the absence of external pressurized fluid supply that significantly simplifies hydrodynamic bearing application for rotating machinery. Moreover due to the flexibility of the foil, such type of bearings are less sensitive to surface finish quality and possible rotating shaft and housing axial misalignment during assembling. Additionally, the use of foil bearings provide desired bearing stiffness characteristics and instabilities dumping.
The pressurized fluid film is formed in the gap between the outer periphery of the rotating shaft and the foil only when the rotating shaft rotates at a certain speed. In order to reach the certain speed, the rotating shaft is to be accelerated from zero rotating speed. At zero rotating speed, as there is no pressurized fluid film in the gap, the outer periphery of the rotating shaft is in direct contact with the inner wall surface of the foil. This leads to dry friction during the start-up and shut-down cycles that inevitably results in intensive surface wear of the foil bearing. Thus, the maximum service life is defined primarily by the strength of the bearing material or in other words by the whole number of start-up and shut-down cycles.